The drivetrain in many motor vehicles includes a power transfer assembly, such as a transfer case, for transmitting drive torque to all four wheels of the vehicle, thereby establishing a four-wheel drive mode of operation. Some transfer cases incorporate a power transfer system that automatically directs drive torque to the non-driven wheels when the driven wheels lose traction. This “on-demand” 4WD feature is accomplished without any input or action on the part of the vehicle operator. Typically, a clutch assembly is interactively associated with an electronic control system for distributing the drive torque between the driven and non-driven wheels. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that drive torque is only delivered to the driven wheels. However, when the sensors detect a low traction condition at the driven wheels, the clutch assembly is automatically actuated to deliver drive torque “on-demand” to the non-driven wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-driven wheels can be varied as a function of specific vehicle operating parameters, as detected by the sensors.
Transfer cases typically are equipped with a mechanical or hydraulic clutch actuation mechanism for controlling the operation of the clutch assembly. Friction that naturally occurs within the clutch actuation mechanism can hinder its performance. For example, sliding friction can limit the precision with which the clutch actuation mechanism can be controlled. Similarly, static friction can increase the response time of the clutch actuation mechanism and may limit the degree to which small incremental changes in the torque distribution between the driven and non-driven wheels can be made. Thus, a recognized need exists for developing a method for operating an adaptive clutch system that will reduce the negative effects that friction has on its performance.